## BS 7671 Voltage Drop Calculation

Last updated on 2017-07-16 2 mins. to read## Contents

```
clc;
clear;
format compact;
```

## Example Cable Calculation BS 7671 Voltage Drop

For calculation details please refer to https://mycableengineering.com/knowledge-base/bs-7671-voltage-drop.

## Cable Details

table = '4E3A Single-core armoured 90 °C themosetting, copper conductors'; CSA = 50; % cross sectional area of cable, mm2 Length = 80; % length of cable, m % Uo = 400; % line-line voltage, V Ib = 152; % load current, A pf = 0.87; % power factor phases = 3; % number of phases (0 = d.c.) tp = 90; % maximum permitted cable temperature, °C

Ensure that we have both cos(ph) and sin(phi) from the power factor.

cos_phi = pf; sin_phi = sin(acos(pf)); disp(''); disp(['pf = cos(phi) = ', num2str(cos_phi), ' which gives sin(phi)= ', num2str(sin_phi, '%0.2f')]);

pf = cos(phi) = 0.87 which gives sin(phi)= 0.49

## BS 7671 Looup Values

It = 222; % tablulated current A, table 4E3A, column 6 mVr = 0.86; % (mV/A/m)r table 4E3B, column 5 mVx = 0.29; % (mV/A/m)x table 4E3B, column 5 Ca = 0.96; % ambient temperature derating, table 4B1 Cg = 1.00; % grouping derating, [no grouping] Cs = 1.00; % soil thermal resistivyt derating [not buired] Cd = 1.00; % depth of burial derating [not buried]

## Calculation

Ct = (230+tp-(Ca^2*Cg^2*Cs^2*Cd^2-(Ib^2/It^2))*(tp-30))/(230+tp); disp(['Temperature correction factor Ct = ', num2str(Ct, '%0.2f' )]); vd = ( Ct*cos_phi*mVr + sin_phi*mVx ) * Ib * Length /1000; vdPercent = 100*vd/Uo; disp('Voltage drop = ', num2str(vd, '%0.2f'), ' V [', num2str(vdPercent, '%0.2f'), '%]');

Temperature correction factor Ct = 0.92 Voltage drop = 10.06 V [2.52%]

### Comments

Feb 19, 2018 8:28 PM

this example is so much bigger than using the voltage drop calculator.

the result from voltage drop calculator is: voltage drop=2.26v, 0.57%.

why?

the result from voltage drop calculator is: voltage drop=2.26v, 0.57%.

why?

Steven McFadyen
Feb 19, 2018 10:33 PM

The calculated voltage drop looks about right. I have just checked with two different software programs, and they are coming up with a very similar answer.

Please, can you provide more information on which voltage drop calculator you used, then I can give a more detailed answer?

Please, can you provide more information on which voltage drop calculator you used, then I can give a more detailed answer?